Chromium-free pickle for plastic surfaces

ABSTRACT

A pickling solution for the surface pre-treatment of plastic surfaces in preparation for metallization, the solution comprising a source of Mn(VII) ions; and an inorganic acid; wherein the pickling solution is substantially free of chromium (VI) ions, alkali ions, and alkaline-earth ions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/672,980, filed Aug. 12, 2010, which issued as U.S. Pat. No. 9,023,228on May 5, 2015 and which is a national stage application ofPCT/US08/72779, filed Aug. 11, 2008, each of which is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a pickling solution and to a processfor pickling plastic surfaces. In particular, the present inventionrelates to a pickling solution and to a pickling process for picklingABS plastic surfaces or ABS polymer blend surfaces prior to a subsequentmetallization of these surfaces.

BACKGROUND OF THE INVENTION

Plastic surfaces are frequently coated with suitable metal layers eitherfor technical reasons or for decorative reasons. In the case oftechnically reasoned coatings the same can be applied for instance forforming electrically conducting structures on the plastic surfaces. Ifthe coating is applied for decorative reasons, plastic surfaces having ahigh quality appearance will be produced in this way. Especially in thefield of automotive construction decoratively metalized plastic surfacesare widely spread. In the domain of electronic and electricalengineering conductive structures are deposited for instance on plasticboards for forming integrated circuits by means of suitable metaldeposition processes.

For the durability of the metal layers deposited on the plastic surfacesit is decisive that the same exhibit sufficient adhesion on the plasticsurfaces. To provide for a corresponding adhering strength, it is thecommon practice in prior art to roughen the plastic surfaces prior to acorresponding metal deposition, in order to be able to provide for asufficient adhering strength of the deposited metal layers. For thispurpose different processes are known from prior art. Typically, plasticsurfaces are treated with chromium(VI) containing pickling solutions.Such solutions which are based on chromium-sulfuric acid can include forinstance chromium(VI) oxide and sulfuric acid at a weight ratio of 1:1.

At the immersion of the plastic surface into such chromium acid picklingsolution the polybutadiene component on the plastic surface willoxidatively decompose and leave cavities on the surface, which serve ascorresponding anchoring structures for the deposited metal layers. It isassumed that the adhesion is produced by this anchoring effect.

On the other hand, chromium(VI) containing compounds are suspected ofbeing cancer-causing, so that dealing with these compounds is subject tostrict environmental regulations. In the face of the potential dangercaused by chromium(VI) releasing compounds, a prohibition of theindustrial use of chromium(VI) containing compounds cannot be excluded.

An alternative for the mentioned chromium(VI) containing picklingsolutions for plastic surfaces are pickling solutions based on alkalipermanganates. For this purpose, pickling solutions based on alkalipermanganates, preferably potassium permanganate, and a mineral acidlike for instance phosphoric acid are used.

The pickling solutions thus prepared are free of chromium(VI) containingcompounds and exhibit a good pickling effect for plastic surfaces,especially plastic surfaces from ABS plastic.

Alkali permanganate-based pickling solutions typically contain up to 20g/l of alkali permanganate in a corresponding mineral acid solution suchas a 40 to 85% phosphoric acid.

However, the alkali permanganate which is used is subject to a rapidself-decomposition, so that the pickling effect of a freshly preparedalkali permanganate pickling solution will be insufficient already afterseveral hours. As a decomposition product of the alkali permanganatescolloidal manganese(IV) compounds are apparently produced which neitherallow filtering nor centrifuging. The pickling solution which has lostits pickling effect can be re-sharpened by the addition of alkalipermanganate. During this however, the colloidal manganese(IV)decomposition products, manganese(II) ions as well as the alkali ionsbecome even more enriched within the pickling solution. As a result ofthis enrichment the viscosity of the pickling solution will be increasedup to a point where the pickling solution must be discarded, becausesatisfying pickling results cannot be achieved any longer.

A further drawback of the described alkali permanganate picklingsolutions is the colloidal appearance of the decomposition products.Since these decomposition products, as described above, can neither befiltered nor centrifuged, these colloidal manganese(IV) compounds arefrequently spread over the entire coating process of the plasticsurfaces.

But this spreading may lead to a strong influence on the depositionprocess, so that metal layers are finally deposited which are faulty orotherwise insufficient.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide achromium-free pickling solution for plastic surfaces which is able toovercome the problems known from prior art in connection with the use ofalkali permanganate pickling solutions. It is also an object of thepresent invention to provide a suitable process for pickling plasticsurfaces, especially surfaces of ABS plastic and ABS blends.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This application claims priority to European application 07015812.6, theentire disclosure of which is expressly incorporated herein byreference.

Concerning the pickling solution, this object is solved by a picklingsolution for the surface pre-treatment of plastic surfaces, includingMn(VII) ions as well as an organic acid, characterized in that thepickling solution is free of alkali and alkaline-earth ions.

As an inorganic acid the pickling solution in accordance with theinvention includes an acid from the group consisting of phosphoric acid,peroxomonophosphoric acid, peroxodiphosphoric acid, sulfuric acid,peroxomonosulfuric acid or peroxodisulfuric acid. In one embodiment ofthe invention also a mixture of at least two of the above-mentionedacids can be used.

The concentration of the Mn(VII) ions contained in the pickling solutionis according to the invention within a range between 0.001 and 0.5mol/l. Here, the concentration of the inorganic acid from the mentionedgroup can amount between 1 and 18.5 mol/l.

In addition to the Mn(VII) ions the pickling solution according to theinvention can include further ions for supporting the pickling process.For this purpose silver, bismuth, vanadium, molybdenum, lead or copperions or mixtures thereof are particularly suited. The concentration ofthese further ions can be within a range between 0.001 and 2 mol/l,preferably between 0.001 and 0.01 mol/l. The addition of such furtherions increases the efficiency and supports the formation of aggressiveetching active substances, for which the brown colour of the picklingsolution is characteristic.

Moreover, the pickling solution according to the invention can includefurther additives such as wetting agents or defoamers.

The Mn(VII) ions contained in the pickling solution are obtained inaccordance with the invention by means of an anodic oxidation inside anoxidation cell from a solution which includes Mn ions of a loweroxidation stage than (VII). The Mn ion containing solution is producedby dissolving or dispersing a suitable manganese salt in an acidsolution. Suitable manganese(II) salts are for instance MnCO₃, MnSO₄,MnO, MnCl₂, Mn(CH₃COO)₂ and Mn(NO₃)₂ or mixtures thereof. Suitablemanganese(III) salts are for instance Mn₂O₃, MnPO₄, MnO(OH), Mn(CH₃COO)₃and MnF₃ or mixtures thereof. A suitable Mn(IV) salt is for instanceMnO₂.

As an acid the acidic solution used for the oxidative production ofMn(VII) ions includes an acid from the group consisting of phosphoricacid, peroxomonophosphoric acid, peroxodiphosphoric acid, sulfuric acid,peroxomonosulfuric acid or peroxodisulfuric acid. In an advantageousembodiment of the invention the acid used in the solution for theoxidative production of Mn(VII) ions corresponds to the acid which iscontained in the pickling solution. In one embodiment of the inventionalso a mixture of at least two of the above-mentioned acids can be used.

The production of Mn⁷⁺ as well as the pickling process according to theinvention can take place inside an oxidation cell. The oxidation cellwhich is used for this purpose includes an anode space and a cathodespace which are separated from each other through a diaphragm or amembrane correspondingly allowing ions to pass through. As an anodemixed oxide electrodes are suited such as an iridium/ruthenium mixedoxide coated titanium electrode for instance. As an alternative alsodiamond coated niobium anodes, anodes from massive platinum orplatinum-plated anodes are suitable. The anode material which is usedmust exhibit a sufficiently high oxygen overvoltage for allowing anoxidation of Mn ions of a lower oxidation stage into Mn(VII). On theother hand, the oxygen overvoltage of anode material must be low enoughto avoid the formation of peroxomonosulphate, peroxidisulphate, andH₂O₂. As a cathode material corrosion-resistant electrodes, for instancesuch consisting of stainless steel or graphite are suitable. Aprecondition for the employed cathode material is that the same isresistant to the acid contained in the catolyte.

The anolyte can consist of a concentrated acid and a Mn ion releasingcompound dissolved therein. A suitable anolyte solution consists forinstance of 0.1 to 0.3 mol/l manganese carbonate in 85% phosphoric acid.

In accordance with the invention, the catolyte can consist of acorrespondingly diluted mineral acid such as for instance 50% phosphoricacid solution. By applying a current density of 0.1 to 100 A/dm²,preferably 1 to 50 A/dm² and even more preferably 2 to 25 A/dm², Mn(VII)ions are galvanically oxidatively formed.

For the oxidative formation of Mn(VII) ions an anode surface of 0.01 to3 dm², preferably 0.1 to 3 dm² per liter of the anolyte turned out assuitable. The anodic oxidation can advantageously be increased in itsefficiency by applying a reverse pulse current.

The pickling solution thus formed in the anolyte exhibits a sufficientpermanganese concentration in order to pickle plastic surfaces such asfor instance surfaces of ABS plastic or ABS blends. As possible counterions of the permanganese ions present in the pickling solution, thepickling solution itself merely contains H⁺ and Mn²⁺ ions.

The pickling solution according to the invention preserves its picklingeffect also at an increased temperature such as e.g. 50° C. for manydays, which fact is indicative of a high stability of the picklingsolution. On the other hand, alkali permanganese pickling solutionscompletely lose their pickling effect within 24 hours. The workingtemperature of the inventive pickling solution can be in the range ofbetween about 20° C. and about 90° C., with a preferred range betweenabout 55° C. and about 80° C.

It can be assumed that not only the permanganese that has been formedcontributes to the pickling effect, but also the fact thatperoxomonphosphoric acid and/or peroxodiphosphoric acid are formed as aresult of the oxidative process in the pickling solution, as well as thefact that the manganese occurs in all oxidation stages between +2 and+7. Indicative of this assumption is the changing color of the picklingsolution from clear magenta in the starting phase of the electrolyticoxidation over dark red to brown in the further progress of the anodicoxidation process. The substance which makes the solution turn to brownturns out to be a stronger oxidant than permanganese acid when subjectto the quantitative analysis by means of ferrometrie with potentiometricindication.

In one embodiment of the inventive pickling solution, phosphoric acid isused in combination with sulphuric acid. The acids were used in a ratioof 80% phosphoric acid to 20% sulphuric acid. The addition of sulphuricacid to phosphoric acid gains to an increase of the electrochemicalefficiency, which results in an increased etching effect.

A further embodiment of the inventive pickling solution, an aqueoussolution of sulphuric acid was used as inorganic acid. The concentrationof the sulphuric acid in this aqueous acidic solution can be within arange of about 800 g/l to about 1400 g/l, with the preferred range ofabout 1100 g/l. The manganese concentration in the sulphuric acidcomprising aqueous solution can be as high as possible, so that thesolution is saturated with manganese at the working temperature. Forexample, in an aqueous solution comprising 1100 g/l H₂SO₄ at atemperature of 65° C. about 91 mmol/l can be dissolved. A preferredsource for manganese is MnSO₄*4H₂O, whereby also manganese salts havinga low oxidation state are usable. By the use of sulphuric acid in theinventive pickling solution exhibits a significant increase of theelectrochemical efficiency. Furthermore, when sulphuric acid is used asonly acid in the inventive pickling solution, the formation of unsolubleMn(III)-compounds is avoided. This reduces the apparative complexity ofthe etching system since there is no excessive need for filtration tomaintain the electrolyte.

Concerning the process, the object of the invention is solved by aprocess for pickling plastic surfaces, comprising a step of contactingthe plastic surface to be pickled with an acidic treatment solution freeof alkali or alkaline earth and including Mn(VII) ions.

In one embodiment of the process according to the invention thecontacting of the plastic surface to be pickled with the acidic Mn(VII)ions containing treatment solution which is free of alkali and alkalineearth takes place inside a device formed by an anode space and a cathodespace, characterized in that the plastic surface to be pickled iscontacted with the acidic Mn(VII) ions containing treatment solutionwhich is free of alkali and alkaline earth inside the anode space orinside a working container which is hydraulically connected to the anodespace, wherein the anode space is filled with an acidic solutionincluding a compound which releases Mn ions of a lower oxidation stageand wherein the two electrode spaces are separated from each other by adiaphragm or a membrane correspondingly allowing ions to pass throughand wherein the Mn(VII) ions contained in the pickling solution areproduced through anodic oxidation by means of applying a suitablevoltage.

Conditional on the self-decomposition and particularly on the picklingof the plastic parts a decrease in the permanganese concentration in thepickling solution is caused. Here, the following equations must be takeninto account:HMnO₄+H₃PO₄→MnPO₄+2H₂O+O₂↑  Equation 1:MnO₄ ⁻+8H⁺+5e−→Mn²⁺+4H₂O  Equation 2:HMnO₄+4MnHPO₄+H₃PO₄+H₂O→5MnPO₄+4H₂O  Equation 3:4HMnO₄+4H₃PO₄→MnHPO₄+6H₂O+5O₂↑  Equation 4:

The Mn²⁺ that has been formed can be oxidized. However, the manganesephosphate that has been formed must be separated, because the same mayblock the anode surface and has a negative influence on the furtherprogress of the coating process. The manganese phosphate that has beenseparated by filtration or other techniques can be converted by means ofan appropriate reducing agent such as oxalic acid, ascorbic acid or alsohydrogen peroxide in a stoichiometric or slightly hypostoichiometricamount. Here, the reducing agent is advantageously added as aconcentrated solution by means of a dosing pump.

Through the addition of the reducing agent the non-soluble Mn(III)phosphate is changed into soluble Mn²⁺ compounds. The Mn(II) compoundsthus obtained can be recirculated as a reactant to the anodic oxidation.As an alternative, the manganese(III) phosphate can also be boiled up ina part of the pickling solution, whereat the non-soluble manganese(III)phosphates are changed into soluble manganese compounds without theaddition of further reagent phosphate.

This advantageously provides for a closed material cycle. It has beenfound out that through the addition of further ions of elements like forinstance Ag, Bi, V, Mo or Cu the efficiency of the anodic oxidationcould be increased.

EXAMPLE 1

An injection-molded component made of commercially available ABS plasticin galvano quality, e.g. “Novodur P2 MC” by Lanxess company, Leverkusen,is pickled at 60° C. for 10 minutes in a solution of 0.3 mol/L Mn²⁺ ionsdissolved in concentrated orthophosphoric acid with w(H₃PO₄)=0.85, inwhich at least 0.025 mol/L of Mn(VII) ions have been anodicallyproduced. By this pickling treatment the surface is chemically etched insuch a way that the caverns necessary for the provision of good adhesionare generated. The thus prepared surface can then be metalized followingthe steps known to one skilled in the art, in a similar way as after thepickling process in a chromium sulfuric acid solution. In thisconnection, all known metallizing systems come into consideration, ofwhich the colloidal activation, the ionogeneous activation and thedirect metallizing systems may be mentioned for example. The depositedlayers are free of delaminations and exhibit a good adhesive strength.

EXAMPLE 2

An injection-molded component made of commercially available ABS plasticin galvano quality, e.g. “Novodur P2 MC” by Lanxess company, Leverkusen,is pickled at 50° C. for 10 minutes in a solution of 0.1 mol/L Mn²⁺ ionsdissolved in sulfuric acid solution with β(H₂SO₄)=1400 g/L, in which atleast 0.010 mol/L of Mn(VII) ions have been anodically produced. Thispickling solution additionally contains 2 mmol/L silver ions which havebeen added as silver carbonate. By this pickling treatment the surfaceis chemically etched in such a way that the caverns necessary for theprovision of good adhesion are generated. The thus prepared surface canthen be metalized following the steps known to one skilled in the art,in a similar way as after the pickling process in a chromium sulfuricacid solution. In this connection, all known metallizing systems comeinto consideration, of which the colloidal activation, the ionogeneousactivation and the direct metallizing systems may be mentioned forexample. The deposited layers are free of delaminations and exhibit agood adhesive strength.

EXAMPLE 3

An injection-molded component made of a commercially available ABS/PCblend in galvano quality, e.g. “Bayblend T45” by Lanxess company,Leverkusen, is pickled at 50° C. for 10 minutes in a solution of 0.1mol/L Mn²⁺ ions dissolved in a sulfuric acid solution with β(H₂SO₄)=1400g/L, in which at least 0.010 mol/L of Mn(VII) ions have been anodicallyproduced. This pickling solution additionally contains 2 mmol/L silverions which have been added as silver carbonate. By this picklingtreatment the surface is chemically etched in such a way that thecaverns necessary for the provision of good adhesion are generated. Thethus prepared surface can then be metalized following the steps known toone skilled in the art, in a similar way as after the pickling processin a chromium sulfuric acid solution. In this connection all knownmetallizing systems come into consideration, of which the colloidalactivation, the ionogeneous activation and the direct metallizingsystems may be mentioned for example. The deposited layers are free ofdelaminations and exhibit a good adhesive strength.

What is claimed is:
 1. A process for pickling plastic surfacescomprising: a) dispersing a manganese salt in an acid solution in anoxidative cell to produce Mn(VII) ions; and b) contacting the plasticsurface to be pickled with an acidic treatment solution, the acidictreatment solution comprising the Mn(VII) ions and an inorganic acid,wherein the inorganic acid is at least one acid selected from the groupconsisting of phosphoric acid, peroxomonophosphoric acid,peroxodiphosphoric acid, sulfuric acid, peroxomonosulfuric acid, andperoxodisulfuric acid; wherein the acidic treatment solution issubstantially free of chromium ions, alkali ions and alkaline earthions; and wherein the acid used in the acid solution of the oxidativecell comprises the same acid as the inorganic acid in the acidictreatment solution.
 2. The process for pickling ABS plastic surfacesaccording to claim 1, wherein the acid treatment solution comprisessilver ions.
 3. The process for pickling ABS plastic surfaces accordingto claim 1, wherein the acidic treatment solution in the oxidation cellis maintained at a temperature in the range of 20° C. to 90° C.
 4. Theprocess for pickling ABS plastic surfaces according to claim 1, furthercomprising the step of metallizing the pickled ABS plastic surfaces. 5.The process for pickling ABS plastic surfaces according to claim 1,wherein the ABS plastic surfaces comprise an ABS plastic or an ABSplastic blend.
 6. A pickling solution for the surface pre-treatment ofplastic surfaces in preparation for metallization, the solutioncomprising: a source of Mn(VII) ions; an inorganic acid, the inorganicacid comprising 800 g/l to 1400 g/l sulfuric acid; wherein the picklingsolution is substantially free of chromium (VI) ions, alkali ions, andalkaline-earth ions; and wherein the formation of insolubleMn(III)-compounds is avoided.
 7. The pickling solution according toclaim 6 wherein the source of Mn(VII) ions provides Mn(VII) ions at aconcentration between 0.001 and 0.5 mol/l.
 8. The pickling solutionaccording to claim 6 wherein the inorganic acid has a concentrationbetween 1 and 18.5 mol/l.
 9. The pickling solution according to claim 6further comprising ions of one or more elements from the groupconsisting of Ag, Bi, V, Mo, and Cu.
 10. The pickling solution accordingto claim 9 further comprising silver ions.
 11. The pickling solutionaccording to claim 6 comprising between 0.001 and 2.0 mol/l ions of oneor more elements from the group consisting of Ag, Bi, V, Mo, and Cu. 12.The pickling solution of claim 6 further comprising a wetting agent. 13.The pickling solution of claim 6 further comprising a defoamer.
 14. Thepickling solution of claim 6 wherein the Mn(VII) ions are obtained bymeans of an oxidation cell from a Mn-ion of an oxidation state lowerthan VII.
 15. The pickling solution according to claim 14, wherein thesolution for the oxidative production of Mn(VII) ions is an acidicsolution.
 16. The pickling solution according to claim 14 wherein thesolution for the oxidative production of Mn(VII) ions includes as asource of Mn ions of an oxidation stage lower than (VII) at least onemanganese compound selected from the group of MnCO₃, MnSO4, MnO, MnCl₂,Mn(CH3COO)₂, Mn(NO₃)₂, Mn₂O₃, MnPO₄, MnO(OH), Mn(CH₃COO)₃ and MnF₃ andMnO₂.
 17. The pickling solution according to claim 16 wherein thesolution for the oxidative production of Mn(VII) ions includes at leastone acid from the group consisting of phosphoric acid,peroxomonophosphoric acid, peroxodiphosphoric acid, sulfuric acid,peroxomonosulfuric acid and peroxodisulfuric acid.
 18. A process forpickling plastic surfaces comprising: a) providing an oxidation cellcomprising an anode space and a cathode space separated from each otherby a membrane capable of allowing ions to pass through; b) disposing ananolyte in the anode space, the anolyte comprising a concentrated acidand a Mn ion releasing compound; c) disposing a catolyte in the cathodespace, the catolyte comprising a diluted mineral acid; d) applying acurrent density to galvanically oxidatively form an acidic Mn(VII)containing treatment solution; and e) contacting the plastic surface tobe pickled with the acidic Mn(VII) containing treatment solution insidethe anode space or a working container connected to the anode space;wherein the acidic treatment solution is substantially free of chromiumions, alkali ions and alkaline earth ions.
 19. The process for picklingABS plastic surfaces according to claim 15, wherein the acidic treatmentsolution further comprising ions of one or more elements selected fromthe group consisting of Ag, Bi, V, Mo, and Cu.
 20. The process forpickling ABS plastic surfaces according to claim 18, wherein the acidictreatment solution in the oxidation cell is maintained at a temperaturein the range of 20° C. to 90° C.